Electrochemical protection of the recesses, narrow slits, small diameter and/or deep holes, grooves, pipes, corners and equivalent of perforated rolls against corrosion, e.g. cathodic protection, is cumbersome, expensive and frequently technically impossible by methods in present use, because usually the protective electric current cannot be directed on those points of the structure which are critical in view of corrosion or soiling. The objects which are particularly embarrassing from the viewpoint of corrosion control or soiling are rotating perforated rolls, e.g. the shell, or body part, of a paper machine suction roll, or a drum in a bleaching filter in a pulp mill.
In the thick-walled shell of a suction roll a great number of small diameter, long holes have been drilled. Unsatisfactory durability of the shell which is perforated and serves in chemically corrosive conditions, and which is physically subjected to heavy load, so-called pinpoint corrosion, stress/fatigue corrosion fractures and soiling have since decades been among the worst problems faced by paper machine manufacturers and papermakers. Although there has been development of the shell materials in recent years, the conditions giving rise to suction roll corrosion have become worse, and soiling has increased, in the first place owing to higher degree of closed water circulation. The heavy, fatiguing load has also increased as the machines have increased in width and the running speeds have gone up. New shell metals which present greater durability than before, for instance two-phase steels, evoke problems because they often carry residual stresses from the manufacturing process, because their drilling is difficult and because they command a high price. Fractures of this new roll type have in fact occurred after unexpectedly short service periods.
Electrochemical anticorrosive protection, by presently employed methods, of perforated rolls in pulp mills, that is, of filter drums, is not fully satisfactory because the electric current cannot be directed on the inner surfaces of the perforated shell, on recesses inside the drum, etc. when stationary electrodes outside the drum are used. Existing procedures also fail to afford satisfactory protection of those parts of the outer drum surface which at any given time are positioned outside and/or above the filtrate in the basin, that is, of the electrolyte, and are in contact with the corrosive pulp slurry; in corrosion trials under laboratory conditions, for instance, electrochemical protection is 100% effective and in plant conditions, 60 to 80% effective, depending on the steel.
A suction roll operates in that water is being drawn from the side of the outer shell periphery, from the paper web, through the holes in the shell and into a suction box on the side of the inner shell periphery. In modern fast paper machines little, if any, water passes through the holes into the suction box, owing to high peripheral velocity: the holes are rather filled when opposite the suction box, and after passing the suction box the water flies out from the holes, by centrifugal effect, to the ambience outside the shell. The region which is problematic regarding corrosion and soiling lies within the holes. Existing technology is unable to protect the surface inside the holes by electrochemical means against corrosion or soiling because electric current cannot be induced to flow in the critical region inside the holes at any high enough current density. In systems conforming to existing technology, the stationary current electrodes, e.g. anodes, have to be disposed outside the rotating shell, where they are susceptible to damage and where they may cause damage when they get loose and end up in the press nip. Furthermore, the density of protective current supplied from outside the shell is not sufficient with a view to cathodic protection.
It is a further fact that since in fast paper machines in practice no water at all passes through the holes in the suction roll, supplying electric current from inside the suction roll shell to the shell with the aid of the water drained from the paper web is an impossibility. One might contemplate the supplying of water, i.e., of electrolyte, on the inner surface, but because of the high peripheral velocity of the shell and the large aggregate hole area the quantity of such water would be excessive, inhibiting the normal operation of the roll. Attempts to supply water from the outer periphery side of the shell are often impeded by the roll coating.